This invention relates to discharge rollers for discharging copy paper out of an image forming apparatus such as a copying machine after the paper is passed between fixing rollers to have images formed on the paper fixed.
In an electrophotographic image forming apparatus such as a copying machine, a visible image (toner image) formed on a recording medium is transferred onto a sheet of image receiving paper and after this paper is transported to a fixing device to have the transferred toner image fixed, it is discharged out of the apparatus, for example, into a discharge tray. For fixing such a toner image formed on a copy paper, use is frequently made of a pair of fixing rollers which are pressed against each other, one of them being what may be referred to as a heat roller which contains a heater inside and the other, which may be referred to as a compression roller, being pressed against the heat roller with an appropriate pressure between them. The heat roller in such a pair is often designed in a so-called reverse crown shape in order to prevent the copy paper from becoming wrinkled when it passes between the heat and compression rollers. According to this design, the roller has a greater diameter at both ends than at the center (by 0.05-0.2 mm, for example) such that the paper passing between the heat and compression rollers tends to travel faster at the edges than at the center. This has the effect of stretching the paper and thereby preventing it from becoming wrinkled.
If the paper which is coming out of the fixing rollers has different speeds at the edges and at the center as explained above, however, it tends to flex unevenly between the pair of fixing rollers and the discharge rollers which are placed at downstream positions with respect to the fixing rollers to guide the paper out of the copying machine. This has the undesirable effect of forming waves in the paper a shown in FIG. 4 with respect to the direction of motion of the paper indicated by the arrow A when it is discharged. Such sheets of copy paper are difficult to sort.
FIGS. 5 and 6 show discharge rollers previously designed for eliminating the difficulties described above. According to this design, a driver roller 1 and a follower roller 2 are provided parallel and adjacent to each other in a side-by-side relationship. A sheet of copy paper 3, which comes out of a pair of fixing rollers (not shown in FIGS. 5 and 6) in a wavy condition, as explained above and illustrated in FIG. 4, is passed between the discharge rollers 1 and 2, and this tends to correct the wavy condition of the paper. In order to improve this correction effect, the distance t.sub.1 indicated in FIG. 5 or t.sub.2 indicated in FIG. 6 may be made smaller. If these distances are made excessively small, however, the paper 3 may become folded.
With discharge rollers shown in FIG. 5 or 6, however, the condition shown in FIG. 4 can be corrected only to a very limited extent. Corrections are far from perfect.